The present invention relates generally to electronic filter circuits and more particularly to an electronic filter circuit which is self-tuning to be resonant at the frequency of an applied input signal. The basic purpose of any filter of this nature is to pass those signals of desired frequency and to reject or block signals of other frequencies. The other frequency signals are often in the form of "noise" or harmonics which are superimposed on the signal desired to be passed. If the fundamental input signal to be filtered is of relatively constant frequency very few problems are experienced in that the bandwidth of the filter may be made relatively narrow thus passing freely a signal of desired frequency while rejecting all other signals. When, however, the frequency of the input signal varies considerably, difficulty is experienced in producing a filter which sharply distinguishes between the desired and the unwanted frequencies. In such situations it is desirable to provide a filter whose resonant frequency varies or "tracks" the frequency of a fundamental input signal.
One example of a usage in which the desired signal has a frequency which varies widely is that of a self-excited generator system. In one type of this kind of system the output of the generator is utilized to excite the generator field through the triggering of suitable switching devices (e.g., thyristors) in the excitation system. The output frequency of the generator is related to its speed which can vary drastically, particularly at start up. The magnitude of the generator output also varies with speed (often as much as 15:1 ). These variations in frequency and magnitude coupled with the fact that the generator output is often nonsinusoidal and inclusive of a relatively large amount of "noise" present serious problems in the derivation of an accurate signal for the triggering of the switching devices. This is particularly true when it is realized that in many instances the noise may result in more than one zero crossing for each cycle of the basic frequency.